In a phased array ultrasound imaging system, an ultrasound transducer includes an array of transducer elements. The system includes a multiple channel transmitter and a multiple channel receiver. In the typical case where the number of channels is less than the number of transducer elements in the array, an electronic switch network connects a selected group of transducer elements to the transmitter and receiver channels. Each transmitter channel causes a selected transducer array element to transmit an ultrasound pulse into an object being imaged, typically the human body. The transmitted ultrasound energy is steered and focused by applying appropriate delays to the pulses transmitted from each transducer array element so that the transmitted energy adds constructively at a desired point. The pulse is partially reflected back to the transducer array by various structures and tissues in the body.
Steering and focusing of the received ultrasound energy are effected in a reverse manner. The reflected ultrasound energy from an object or structure arrives at the array elements at different times. The received signals are amplified and delayed in separate receiver channels and then summed in a receive beamformer. The delay for each channel is selected such that the receive beam is focused at a desired point. The delays may be varied dynamically so as to focus the beam at progressively increasing depths along a scan line as the ultrasound energy is received. The transmitted beam is scanned over a region of the body, and the signals generated by the beamformer are processed to produce an image of the region.
Various scan patterns, or scan formats, are known in the prior art. In a linear scan pattern, ultrasound energy is transmitted and received along multiple parallel lines which originate at different points on the transducer array. The parallel lines may be perpendicular to the array or may be steered to a desired angle. The linear scan pattern produces a relatively high quality image, because all scan lines are incident upon structures being imaged from the same direction. The field of view obtained with a linear scan pattern is the same at all depths.
In a sector scan pattern, ultrasound energy is transmitted and received along sector lines which originate at a common apex, typically located on the transducer array. The sector scan pattern has a field of view that increases with depth. A disadvantage of the sector scan pattern is its relatively small field of view at shallow depths. Techniques for increasing the field of view of a sector scan pattern at shallow depths involving shifting the sector scan pattern to originate from a virtual apex located behind the transducer array are well known. The virtual apex scan patterns do not have the advantages of a linear scan in producing a high quality image.
A curved linear scan pattern is similar to a linear scan pattern in that the scan lines originate at different points along the transducer array. However, the face of the transducer array is curved rather than flat, or linear. The field of view of a curved linear scan pattern increases with depth at an angle that is limited by, though not necessarily equal to, the angle of arc subtended by the array.
A scanning technique which utilizes a combination of linear scan and sector scan is disclosed in U.S. Pat. No. 4,664,122 issued May 12, 1987 to Yano. In the disclosed transducer array, the scan format is realized by making the spacing between elements in subarrays at each end of the array less than the spacing between elements in a central subarray. Steering of the linear scan is not disclosed.